Ultrafine powders and their use as lasing media

ABSTRACT

Doped, nanosize metal oxide particles have been shown to exhibit stimulated emission and continuous-wave laser action when energized appropriately, for example by electron beams. The doped particles are useful as solid state lasing devices and “laser paints”. Particles containing homogeneously distributed dopant atoms in concentrations greater than the thermodynamic solubility in the metal oxide matrix, and having in some circumstances, unusual oxidation states, have been produced.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. national phase of PCT application numberPCT/US99/28270, filed Nov. 30, 1999, which further claims the benefit ofU.S. provisional application serial No. 60/110,479, filed Dec. 1, 1998.

This invention was made in part with Government support under Grant No.F49620-98-1-0189 awarded by the Air Force Office of Scientific Research.The Government has certain rights in this invention.

TECHNICAL FIELD

The subject invention pertains to solid state powder lasers andnanophosphors, to processes for producing powders suitable for such use,and to the powders produced thereby.

BACKGROUND ART

Past attempts to utilize highly scattering solid materials as candidatesfor stimulated emission have required irradiation with high intensityenergy sources, such as lasers, to demonstrate stimulated emission.“Laser paints”, surfaces coated with solid particulates, have requiredsuch a high threshold level of optical energy input, because of theexcessive attenuation (and therefore loss) that normally accompaniesscattering, that their suitability for most practical purposes is highlyquestionable.

Although electron beams are known to cause emission in solid, dielectricphosphors, these materials are not known to support continuous-wavelaser action. Atoms in the solid lattice that absorb the electron energysubsequently release it spontaneously and randomly. A large portion ofthe energy emitted is reabsorbed by the particulate, by its neighbors,or is highly scattered. Phosphors used in CRT tubes, for example, absorbelectron beam energy and display spontaneous but not stimulatedluminescence, scattering, etc.

Since the discovery of solid state lasers in the early 1960s, lasershave become progressively more commonplace in society, findingapplications ranging from compact disc systems and supermarket scannersto precision surgical, optimetric and cutting instruments. The availabletypes of lasers are very diversified, and now comprise solid, liquid,gas and plasma media pumped by light, electrons, chemical reactions, orother means. Generally, lasers require an external cavity to operate.

Luminescence in the multiple-scattering regime has been reported byseveral researchers from powders containing rare earth and transitionmetal ions, and extensively investigated theoretically. B. M. Tissue,“Synthesis and luminescence of lanthanide ions in nanoscale insulatinghosts,” Chem Mater. 10, 2837-45 (1988), and laser action, N. M. Lawandy,R. M. Balachandran, A. S. L. Gomes and E. Sauvain, “Laser action instrongly scattering media,” Nature 368, 436-438 (1994); C. Gouedard, D.Husson, C. Sauteret, F. Auzel, and A. Migus, “Generation of spatiallyincoherent short pulses in laser-pumped neodymium stoichiometriccrystals and powders,” J.O.S.A. B10, 2358-2363 (1993); D. Wiersma and A.Lagendijk, “Light diffusion with gain and random lasers,” Phys. Rev.E54, 4256-4265 (1996); V. S. Letokhov, “Generation of light by ascattering medium with negative resonance absorption,” Sov. Phys. JETP26, 835 (1968); S. John and G. Pang, “Theory of lasing in amultiple-scattering medium,” Phys. Rev. A54, 3642-3652 (1996). Spectralnarrowing and threshold behavior have been reported in the absence ofexternal cavities, and measured transient behavior shown to becharacteristic of inverted systems of impurity ions with opticalfeedback mediated entirely by scattering. Lossless powders in which gainis encountered despite very short scattering mean free path lengths aresometimes referred to as “laser paints.” D. Wiersma and A. Lagendijk,“Laser action in very white paint,” Physics World, 33-37, January 1997.

Laser paint media in which the mean transport length l* is actually lessthan the wavelength itself have interesting properties which may beuseful for speckle-free lithography at sub-micron dimensions orapplications in which bright, omni-directional output is desired fordisplays or light sources of arbitrary shape. However, highly-scatteringpowders are difficult to pump and study optically because the veryscattering that provides the feedback for laser action causes pump lightto be scattered backwards very efficiently as it enters the medium.Incident light does not penetrate the medium well and the overallefficiency of any pumping and lasing processes is destined to be low.This effect would be particularly true under conditions of “stronglocalization,” when light propagation undergoes an Anderson transition,P. W. Anderson, “Absence of diffusion in certain random lattices,” Phys.Rev. 109, 1492 (1958); P. W. Anderson, “The question of classicallocalization: a theory of white paint?,” Philos. Mag. B52, 505 (1985),to a regime of recurrent scattering which results in completelylocalized electromagnetic “transport.” D. S. Wiersma, M. P. van Albada,B. A. van Tiggelen, and A. Lagendijk “Experimental evidence forrecurrent multiple scattering events of light in disordered media,”Phys. Rev. Lett. 74, 4193-4196 (1995). In this regime, scattering wouldbe so strong that the direction of light would be randomized before ithas propagated a distance of even one wavelength.

Previous experiments, and theory, on localization of light andstimulated emission in scattering media, as well as interest in theconsequences of recurrent scattering events have all heightened currentinterest in electromagnetic phenomena in multiple-scattering media. See,e.g., M. P. Van Albada and A. Lagendijk, Phys. Rev. Lett. 55, 2692(1985); V. M. Markushev, V. F. Zolin, and Ch. M. Briskina, Sov. J. Qu.El. 16, 281 (1986); A. Z. Genack and N. Garcia, Phays. Rev. Lett. 66,2064 (1991); J. X. Zhu, D. J. Pine, and D. A. Weitz, Phys. Rev. A44,3948-3959 (1991); C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A.Migus, J.O.S.A. B10, 2358-2363 (1993); N. M. Lawandy, R. M.Balachandran, A. S. L. Gomes and E. Sauvain, Nature 368 436-438 (1994);M. Siddique, R. R. Alfano, G. A. Berger, M. Kempe, and A. Z. Genack,Opt. Lett. 21, 450 (1996); M. A. Noginov, N. E. Noginov, H. J.Caulfield, P. Venkateswarlu, T. Thompson, M. Mahdi, and V. Ostroumov,J.O.S.A. B13, 2024 (1996); D. Wiersma and A. Lagendijk, Physics World,33-37, January 1997; S. John, Phys. Rev. Lett. 53 2169 (1984); P. W.Anderson, Phil. Mag. B52, 505 (1985); V. S. Letokhov, Sov. Phys. JETP26, 835. (1968); S. John and G. Pang, Phys. Rev. A54, 3642-3652 (1996);R. M. Balachandran, N. M. Lawandy, and J. A. Moon, Opt. Lett. 22, 319(1997); D. Wiersma and A. Lagendijk, Phys. Rev., E54, 4256-4265 (1996);G. A. Berger, M. Kempe, and A. Z. Genack, Rev. E56, 6118 (1997); D. S.Wiersma, M. P. van Albada, B. A. van Tiggelen, and A. Lagendijk, Phys.Rev. Lett. 74, 4193-4196 (1995); 18. D. S. Wiersma, P. Bartolini, A.Lagendijk and R. Rhigini, Nature 390, 671 (1997). Traditionally,multiple scattering has been of interest to researchers studyingstatistical aspects of weakly localized light coherence or imaging. Seee.g., M. Kaveh, M. Rosenbluh and I. Freund, Nature 326, 778 (1987), G.Gbur and E. Wolf, Opt. Lett. 24, 10 (1999); E. Wolf, T. Shirai, G.Agarwal, L. Mandel, Opt. Lett. 24, 367 (1999); E. Leith, C. Chen, H.Chen, Y. Chen, D. Dilworth, J. Lopez, J. Rudd, P.-C. Sun, J. Valdmanis,and G. Vossler, J.O.S.A. A9, 1148 (1992). Others have demonstratedpowder lasers in the diffusive propagation regime. See, V. M. Markushev,V. F. Zolin, and Ch. M. Briskina, Sov. J. Qu. El. 16, 281 (1986); C.Gouedard, D. Husson, C. Sauterei, F. Auzel, and A. Migus, J.O.S.A. B10,2358-2363 (1993); M. A. Noginov, N. E. Noginov, H. J. Caulfield, P.Venkateswarlu, T. Thompson, M. Mahdi, and V. Ostroumov, J.O.S.A. B13,2024 (1996). However, at the boundary between the diffusive and strongscattering regimes it is expected that significant changes will occur inthe interaction of light with matter which are fundamentally new. Severescattering is predicted to cause strong Anderson localization of lightwhen absorption is negligible. Three-dimensional confinement of lightwithin regions of sub-wavelength dimensions could have profoundimplications for the degree of coherence and back-scattered intensity ofelastically-scattered light. The dielectric constant becomes difficultto define when propagation is restricted to sub-wavelength “transport”distances (l*<1) in random media, because fluctuations in the structureof the medium and non-uniformities in the field amplitude occur on thesame distance scale. See, S. John, Phys. Rev. Lett. 53, 2169 (1984); 11.P. W. Anderson, Phil. Mac. B52, 505 (1985); R. W. Boyd and J. E. Sipe,J.O.S.A. B11, 297 (1994). Additionally, it has been suggested that theonset of recurrent (closed loop) scattering events in this regime willreduce the threshold for laser action mediated purely by scatteringfeedback. S. John and G. Pang, Phys. Rev. A54, 3642-3652 (1996); and G.A. Berger, M. Kempe, and A. Z. Genack, Rev. E56, 6118 (1997).

DISCLOSURE OF INVENTION

The present invention pertains to solid state lasers composed of metaloxide and mixed-metal oxide nanosize phosphor powders, which arepreferably doped with one or more transition metals, lanthanide metals,or actinide metals. The present invention also pertains to objectscomposed of such nanosize particles, which are capable of lasing whenstimulated by a suitable stimulation method, particularly by impingementof electrons, including a current or beam of electrons. The objects arepreferably in the form of a thin film, or waveguide, and may be suitablefor use as a light source, as a flat panel display component, or forother purposes where emission of light by stimulated emission would beuseful.

The subject invention further pertains to a process for emittingelectromagnetic radiation by stimulated emission, this processcomprising assembling a plurality of doped metal oxide nanoparticles,and exposing the nanoparticles to a suitable excitation source,whereupon stimulated emission of electromagnetic radiation results. Thesource may be a particle beam or current of charged particles, a sourceof radiant energy such as a laser or flashlamp, or a chemical orexplosive reaction. The excitation energy source preferably comprisesenergetic electrons:

The subject invention further pertains to a process for emitting whitelight by stimulating an assembly of doped nanoparticles which emit red,blue, and green or other combinations of primary colors which may beviewed as substantially “white” light. This emission may comprise thejoint emissions of several differently-emitting dopant ions innanoparticles, (mixed within each nanoparticle or residing in separatenanoparticles of a mixed powder), or may be the emission of a single ionspecies undergoing stimulated emission on more than one transitionsimultaneously.

The subject invention further pertains to the preparation of highlydoped nanoparticles whose dopant concentration exceeds thethermodynamically stable concentration limit, by flame pyrolyzing aceramic precursor solution containing a dopant concentration in excessof the thermodynamically stable dopant concentration in the solidceramic.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a-c) illustrates emission intensity and frequency distributionof emission by phosphors irradiated with electron beams in vacuo atvarious current levels (a) Pr: β-Al₂O₃; (b) Nd: δ-Al₂O₃; (c) Ce:δ-Al₂O₃.

FIGS. 2(a-d) illustrates threshold intensities at which the slope ofemissive output abruptly increases as electron current is increased,indicating laser action. (a) Pr: β-Al₂ ₃ at 633 nm. (b) Nd: δ-Al₂O₃ at467 nm. (c) Ce: δ-Al₂O₃ at 362 nm. (d) Nd: δ-Al₂O₃ at 870 nm. The insetin (c) shows the reduction of emission linewidth in Ce: d-Al₂O₃ ascurrent is increased, revealing the onset of modest gain narrowing.

FIGS. 3(a-c) illustrates coherent back-scattered intensity distributedversus angle near the exact back-scattering direction. The detectorscanning plane was perpendicular to the incident polarization. (a) Pr:β-Al₂O₃. (b) Nd: δ-Al₂O₃. (c) Ce: δ-Al₂O₃.

FIG. 4 Emission intensity at room temperature versus electron beamcurrent for Pr: b-Al₂O₃ phosphor particles irradiated at various beamvoltage levels. The inset illustrates the change in emission spectrum.

FIG. 5 Monte Carlo simulation of electron penetration of energeticelectrons into alumina (calculated by the CASINO routine).

FIG. 6 is a plot of l* versus wavelength showing the range ofwavelengths over which strong localization is believed to extend. Stronglocalization, and hence laser action, should occur at any wavelengthwhere l* is substantially less than the wavelength—hence at any l* wherethe solid curve falls below the straight line.

FIG. 7 is a scanning electron micrograph of Ce-doped δ-aluminananoparticles.

BEST MODE FOR CARRYING OUT THE INVENTION

Applicants have discovered that nanosized, doped, metal oxide andmixed-metal oxide particles, particularly those having diameters of lessthan 150 nm, more particularly less than 50 nm, and most preferably inthe range of 1 nm to 35 nm, can exhibit stimulated emission andcontinuous-wave laser action at ultraviolet, visible and infraredwavelengths at relatively low threshold energy levels. The wavelengths,bandwidths, and intensity can be varied by altering the concentrationand nature of dopant atoms, the oxide matrix, particle size, and byincluding ions other than oxide ions in the substantially metal oxideparticles.

The nanosized particles may be prepared by any method which allow thepreparation of metal oxide particles in the cited size ranges, and withthe required dopant content, but are preferably prepared by flame spraypyrolysis and similar techniques which produce distinct (non-aggregated)particles. Because of the rapid, high temperature pyrolysis, levels ofdopants which are higher than typical of thermodynamically stablematerials or which are thermodynamically unstable can be obtained; inother words, the dopant concentration is kinetically controlled.Moreover, and again due to the rapid, high temperature pyrolysis,particulates containing dopant metals having unusual oxidation states,and which thus exhibit electronic transitions which do not normallyoccur, may be obtained. Nanosize particles having dopant concentrationsbeyond the equilibrium amount have not been heretofore disclosed.

A further unusual aspect of the present nanosized particles is that thestimulated emission may be of a wavelength which is longer than theparticle diameter. Emission of light in the UV, visible, and infraredportions of the spectrum has been demonstrated. Simultaneous emission oflight in the red, blue, and green portions of the spectrum is commonlyused to form white light of a desired Color Rendering Index (CRI) basedon the CIE standard of 1931, and has also been demonstrated. Pr-, Tb-,and Nd (or Tm)-doped dielectric nanophosphors, for example, furnish thered, green, and blue components respectively for white light phosphors.In addition, one-component nano-powders such as Ce: YAG that are usedcommercially. in large particle size as white light phosphors areexpected to exhibit stimulated emission of white light.

The nanosized particles exhibit relatively high emission rates at lowinput energies compared to phosphors that operate by spontaneousemission. This can be seen most clearly in FIG. 4. The slopes of theemission curves at 60, 35 and 25 μA where stimulated emission is takingplace for the data sets obtained at 2, 3 and 4 keV respectively-are muchhigher than at 1 μA where only spontaneous emission is taking place.More than one order of magnitude increase in emission rate can beobtained at voltages of 2, 3 and 4 keV, with respect to the low valuesobserved at current levels below threshold. This represents asignificant increase of brightness which may be useful in CRT displays,flat panel displays, in combination with organic light emitting displaysand conducting polymer and conducting transparent ceramic displays,television screens, etc. As shown in FIGS. 2 and 4, emission may besubstantially linear both above and below a threshold beam current.However, the rate of intensity change abruptly differs on either side ofthe threshold.

Stimulation of the nanoparticles can be achieved by optical pumping andother means, but most surprising is that stimulation by impingingelectrons from electron beams, or other electron sources (e.g., anelectric current) can be effective. Electrons do not penetrate far intoinsulating materials such as oxide powders. However, by an unexpectedcoincidence, electrons in the 1-4 keV range penetrate a distancesomewhat larger than the average transport (propagation) distance in thepowder samples of the subject invention. See FIG. 5. Hence theelectron-pumped volume and the volume in which the electromagneticenergy resides are essentially the same, and high efficiency should bepossible. Whether the host is insulating or non-insulating should makelittle difference to light sources like those of the present inventionin which the light originates within a wavelength of the surface, aregion accessible in virtually all materials by electrons of modestenergy in the keV range. This mode of stimulation also enables energystates much higher than those generated by optical pumping toparticipate in the lasing process. Consequently laser action atpreviously unknown laser wavelengths, such as those of the Nd-dopedsamples shown here, can (and does) occur.

To applicant's knowledge, continuous-wave or pulsed lasing activity inregimes where the elastic scattering length l_(SC) and the mean freetransport distance l* are both substantially shorter than the wavelengthemitted have not been previously disclosed. It is believed that thesetwo requirements may be necessary for continuous-wave laser actionmediated by strong scattering, particularly at any reasonable thresholdlevel. Analysis of our measurements (FIG. 3) to determine experimentalvalues of 1* are shown in FIG. 6 and indicate that this condition can beachieved over a wide range of wavelengths. Moreover, it is anticipatedthat the center wavelength of the localization range and the width ofthe range can be modified by changing particle size and composition.Thus, the invention further pertains to stimulated emission from aplurality of particles assembled in such a way that the mean scatteringlength and transport mean free path are less than the wavelength oflight emitted. In one published report, H. Cao et al. (Physical ReviewLetters 82, 2278 (1999) claim to have demonstrated “Random Laser Actionin Semiconductor Powder”. However the disclosed spectral output of thelaser is inconsistent with randomized properties, showing strong angledependence and the appearance of preferentially amplified frequencycomponents or modes. These characteristics show that their laser is notat all “random”. The device described by Cao et al. is believed to be apulsed, optically-pumped laser which requires a much higher thresholdenergy density to operate than the particle lasers of the presentinvention, indicating an altogether different feedback mechanism thanaccounts for the operation of the present devices.

Thus, the present invention pertains to a simple, highly efficientmethod of generating ultrafine (2-500 nm diameter) and fine (1-20 μmdiameter) single metal oxide and mixed-metal oxide powders, doped withvariable quantities of transition, actinide and/or lanthanide metalssuch that the resulting powders are of use in devices that exhibit novelluminescence and in selected instances stimulated emission and/or lasingof infrared, visible and/or UV light either selectively orcoincidentally. In some instances, the resulting particles containhomogeneous concentrations of dopant metal atoms that are far above thenormal, thermodynamic solubility limit for those metals in the matrix.For selected metals, the oxidation state of the metal is not thattypically seen in oxide matrices, which offers the potential for novelluminescent behavior. The type of emission and the wavelength can becontrolled by changing: (1) the type of matrix oxide, (2) modifying thematrix oxide to contain some amount of fluoride, sulfide, phosphide ornitride or combinations thereof, (3) the dopant element, (4) by usingcombinations of dopants, and/or (5) by suitable combinations of dopants,matrices and codoped elements. The resulting powders can be stimulatedin various ways including electron beam, electron current, plasma,optical, or chemical methods to act as phosphors and/or laser paints(random coherent emitting materials).

The lasing powders are preferably produced at rates of 100s of grams tokilograms of ceramic powder per hour. The preferred process ofpreparation involves dissolving metal compounds, including, but notlimited to alkoxides, carboxylates and/or selected compounds and/ortheir salts in alcohol or other flammable solvent and creating anaerosol mist of this solution by using oxygen or air jets. The resultingaerosol mist is passed through a heat source that ignites the aerosol.The flammable solvent and any organic or combustible or volatileinorganic ligands are burned away from the metal(s) creating nanosized,nearly unagglomerated oxide powders that are compositionally close tothe starting solution compositions and frequently single crystalmaterials. This process is called flame spray. pyrolysis (FSP). AlthoughFSP is the preferred method; practitioners of the art will recognizethat this is. only one of many ways that might be practiced to producerelated powders. FSP methods are preferred because they provide accessto kinetic products that are not normally accessible by other methods.

To produce transition metal, actinide and/or lanthanide doped fine andultrafine metal oxide powders by flame spray pyrolysis, it is preferredthat the precursors be soluble in organic solvents and that the metalsform molecular interactions. Metal alkoxides, especially thosestabilized through the use of multidentate chelates, e.g.triethanolamine and triisopropanolamine, e.g., alumatranes, arepreferred because the chelation provides enhanced stability towardshydrolysis and provides an excellent platform for forming double andtriple alkoxides as in Laine, et al. U.S. Pat. No. 5,958,361 (Sep. 28,1999), incorporated herein by reference. However, the use of metalcarboxylates, especially those that are soluble in organic solvents andin particular those that form mixed-metal carboxylates can also be usedwhen desirable alkoxide complexes are not available or areinsufficiently stable for FSP purposes. Both alkoxides and carboxylatesare especially preferred because they are easily and inexpensively madefrom a wide variety of starting materials. In situations where neitheralkoxides nor carboxylates are available or are to difficult to workwith, then metal nitrates can be used, or a variety of organometalliccompounds. However, both these options are less preferable, as the metalnitrates can form explosive mixtures with organic solvents, andorganometallic compounds are costly and sometimes not stable for longperiods of time. Metal chlorides are not preferred because of the toxic,corrosive and polluting nature of the flame pyrolysis byproducts.

The preferred solvents are butanol, methanol, and most especiallyethanol because of its relatively high fuel value, e.g. as compared tomethanol. However other solvents can also be used as well as solventmixtures. The preferred reaction temperatures are 500-2500° C. with themost preferred temperatures being at about 900-2000° C. The solidsloading in solution ranges from 0.1 to 30% with the preferred loadingbeing 1-20% and the most preferred being in the range of 1-10% byweight.

The preferred feed rates are dependent on the size of the combustionchamber and the solids loading of the solution. For systems such as thatdescribed in U.S. Pat. No. 5,958,361, the feed rates can be from about 1to 100 ml/min with the preferred rate being 5-50 ml/min with the mostpreferred rate being about 10-40 ml/min. Much faster feed rates arepossible with lower loadings and conversely slower feed rates arepreferred with higher loadings.

The preferred composition for the doped nanopowders are for the dopantlevels to be between 0.1 ppm and 20 mole percent of the matrix material,with the preferred range being between 0.1 and 500,000 ppm. In someembodiments it is preferred that two or more dopants be usedsimultaneously.

The preferred matrix particle sizes can be between 1 nm and 500 nm withmore preferred ranges being between 1 and 100 nm and the most preferredrange being between 1 and 30 nm, especially if quantum confinementproperties are desired.

The preferred matrix materials can be alumina derivatives (e.g.δ-alumina, α-alumina, β-alumina, yttrium aluminum garnet, spinel, etc.),rare earth oxides including yttria, silicates (e.g. fosterite, mullite,etc.) zinc oxide, indium and tin oxides and indium tin oxide, andmixtures of these metal oxides. Particularly preferred matrix materialsare those that form easily by FSP including δ-alumina, α-alumina,β″-alumina, yttria, and spinel. In some instances, it may be beneficialto pretreat these matrices with ammonia or sulfides or fluoridecontaining materials to modify the oxide matrix to contain nitrogen,sulfide or fluoride, including systems where the oxygen content isminimal, e.g., (Y_(0.919)Yb_(0.08)Tm_(0.001))₂S₃.

Previous efforts have demonstrated “laser paint” action in dielectricmicron-sized oxide powders using high-energy, short optical pulsepumping. The present inventors have used electron pumping to demonstrate“laser paint” emission in nanosized, doped dielectric powders. Thepresent inventors have also examined strong scattering conditions tosearch for evidence that the onset of recurrent scattering trajectories,which begin and end on the same site, facilitate laser action, S. Johnand G. Pang, “Theory of lasing in a multiple-scattering medium,” Phys.Rev. A54, 3642-3652 (1996), and localization. D. Wiersma and A.Lagendijk, “Light diffusion with gain and random lasers,” Phys. Rev.E54, 42564265 (1996).

As result of intensive experimentation, continuous-wave “random” lasersources have been demonstrated. Both ultraviolet and visible laserradiation in the strong scattering regime have been demonstrated in dry,rare earth nano-powders excited by a low current, low voltage electronbeam. By contrast, earlier reports of pulsed laser action in powderswere all optically-pumped experiments in rare earth material in thediffusive regime V. M. Markushev, V. F. Zolin, and Ch. M. Briskina, Sov.J. Qu. El. 16, 281 (1986); C. Gouedard, D. Husson, C. Sauteret, F.Auzel, and A. Migus, J.O.S.A. B10, 2358-2363 (1993); M. A. Noginov, N.E. Noginov, H. J. Caulfield, P. Venkateswarlu, T. Thompson, M. Mahdi,and V. Ostroumov, J.O.S.A. B13, 2024 (1996) or in semiconductor powdersexhibiting frequency selectivity within the transition bandwidth H. Cao,Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chang,Phys. Rev. Lett. 82, 2278 (1999). Observations of laser modes within thetransition bandwidth and angular variation of the mode distributionhowever rule out the possibility that the recurrent scattering pathsresponsible for lasing are from regions of the random medium smallerthan hall a wavelength in size. When recurrent scattering providesfeedback for lasing from source volumes less than (λ/2)³, theconstructive and destructive interference of light required to producefrequency selectivity in the optical range cannot occur. For effectivelaser source volumes smaller than this, the stimulated emission spectrumshould occupy the full gain bandwidth, be independent of viewing angle(because of directional randomization from scattering) and exhibit nospeckle, consistent with strong localization and with truly random laserproperties. In the phosphors described in the present invention allthese unusual features, together with conventional indicators of laseraction such as sharp thresholds, gain-narrowing, linear output andquenching of competing transitions from the upper state, have beenobserved.

It was first necessary to develop methods of synthesizing doped,preferably lanthanide doped, nanosized powders, preferably alumina-basedpowders, by FSP. The methods of FSP production are described in U.S.Pat. No. 5,958,361, incorporated herein by reference, and describe thegeneral methods for producing oxide nanopowders. The FSP method wasmodified by employing ethanol soluble and ethanol stable alkoxideprecursors of the matrix oxides, and mixing ethanol soluble rare earthsalts to provide dopant atoms. In addition, ethanol soluble metalcarboxylates and other compounds can also be used in place of alkoxides.Finally, although not preferable because of cost, handling difficulties,and potential toxicity and pollution problems, a variety oforganometallic and nitrate precursors to the matrix oxides can also beused. The methods employed in the synthesis are general examples thatare not meant to be limiting.

To be suitable for use as lasing particles according to the subjectinvention, the particles must be smaller than 1.5 μm, and generally mustrange in size from somewhat less than 1 nm to about 150 nm, morepreferably in the range of 1 nm to 50 nm, with a range of 1 nm to 30 nmbeing preferred. In addition to the metal oxide or mixed-metal oxide,each particle, on average, must contain at least one dopant atom. Theactual number of dopant atoms will vary with the size of the particles,and may range, for example from 1 or 2 atoms per particle for particlesizes less than 10 nm to 10⁴ or more dopant atoms in 100 nm particles.Larger particles will generally contain yet more dopant atoms.

The metal oxides are any metal oxides that which can be processed intoparticles of the requisite size and dopant concentration. Mostpreferably, the metal oxides are those which can produce particles byflame spray pyrolysis and like processes, for example metal oxides whichcan be prepared from precursors which are soluble in flammable solvents,and which, upon combustion, generate relatively pure oxides. Mostpreferably, the metal oxides are selected from aluminum oxide andyttrium oxide, and mixtures thereof. However, oxides such as boronoxide, titanium dioxide, tin oxide, lead oxide, indium oxide, ytterbiumoxide, silicon dioxide, and other oxides and mixtures thereof are othersuitable candidates. In general, the oxides should be those which whenproduced by pyrolysis in FSP, produce a particle having a well-definedcrystal lattice. However, amorphous and mixed amorphous/crystallinelattices may also be suitable.

The dopant atoms are those which, when the particles are subjected toappropriate energy input, result in stimulated emission. Preferreddopant atoms are transition metal, lanthanide and actinide metal ions.Preferred are all the rare earth ions (RE) including Ce, Eu, Tm and Pr.Other examples are mixtures of dopant atoms, for example(Y_(0.919)Yb_(0.08)RE_(0.001))₂O₃ where RE could be any of the otherrare earth ions, with the exception of Gd³⁺ which has no low-lyingstates able to accept energy from Yb³⁺ (Note: conversely Yb³⁺ has nointermediate states to facilitate acceptance of energy from thehigh-lying ⁶P_(7,2) state of Gd³⁺). Trivalent Tm, Er, Ho, Sm, and Pr areparticularly suitable as co-dopants with Yb.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLES Example 1

In a 2 L round bottom flask, 199.9 g (2.190 mol) of Al (OH)₃.0.72H₂O,326.7 g (2.190 mol) triethanolamine (TEA) and 1 L ethylene glycol (EG)are mixed with constant stirring (magnetic). The reaction is heated to≈200° C. to distill off EG and byproduct water. After ˜2 h, the reactionturns clear, indicating that formation of a soluble [Al(TEA)]₃ complexis complete. Then, 13.71 g (0.343 mol) of NaOH and 2.84 g (0.068 mol) ofLiOH.H₂O are added, and the reaction is refluxed for 1 h. The metalmolar ratio used (Na:Al:Li=1.67:10.67:0.33) represents the previouslydetermined optimum doping level. The reaction is then distilled underN₂, first to remove by-product water then excess EG, until the residueis too viscous to stir. On cooling, a glassy, orange/yellow solid isrecovered from the flask and diluted with EtOH to create a 1-8 wt %residual ceramic solution. To this solution between 0.975 g (0.0022 mol)and 0222 g (0.0005 mol) of thulium nitrate pentahydrate was added. Thedoped solution was converted to an ultrafine oxide by flame spraypyrolysis.

Example 2

Aluminum tri-sec-butoxide (Aldrich) was purified by distillation undervacuum. In a N₂ Dri-Box, the purified aluminum tri-sec-butoxide, 250 g(1.015 mol) was slowly added to a stirred mixture to 155 g (1.039 mol)triethanolamine and 250 ml isopropanol. Within 2 h, the triethanolaminedisplaces the sec-butanol to form an air and water stable alumatrane.The resultant alumatrane in sec-butanol and isopropanol was dilutedwith. ethanol to create a 2 wt % residual ceramic solution. To thissolution 0.250 g (0.0006 mol) of cerium nitrate hexahydrate or 0.250 g(0.0006 mol) of gadolinium nitrate hexahydrate or 0.250 g (0.0006 mol)of neodymium nitrate hexahydrate or 0.250 g (0.0006 mol) of holmiumnitrate pentahydrate or 0.250 g (0.0006 mol) of europium nitratepentahydrate or 0.250 g (0.0006 mol) of terbium nitrate pentahydrate or0.250 g (0.0006 mol) of thulium nitrate pentahydrate was added. This isequivalent to 1000 ppm dopant level. The doped solution was converted toan ultrafine oxide by flame spray pyrolysis. The visible and infraredemissions spectra of Nd³⁺: δ-Al₂O₃ (FIG. 1(b), exhibit laser thresholdsof approximately 20 and 60 μA respectively, as indicated in FIGS. 2(b)and 2(d).

Example 3

Particles of δ- (or β″-) alumina were synthesized by flame spraypyrolysis of metallo-organic precursors, and a dopant level of 1000±100ppm Ce³⁺ (or Pr³⁺) ions was easily achieved. An alumatrane(N(CH₂CH₂O)₃Al) precursor was used, consisting of alumatrane, 2 wt %Al₂O₃ and 0.003% CeO₂ or PrO₂ in ethanol, and produced powders bycombustion A. C. Sutorik, S. S. Neo, T. flinklin, R. Baranwal, D. R.Treadwell, R. Narayanan, and R. M. Laine, J. Am. Ceran. Soc. 81,1477-1488 (1998); R. M. Laine, K. Waldner, C. Bickmore, D. Treadwell,U.S. Pat. No. 5,614,596 (March 1997) at a rate of 50 g/hr. Particlesizes and concentrations were estimated using BET S. Brunauer, P. H.Emmett, and E. Teller, J. Am. Chem. Soc. 60, 309(1938) surface areas(79.5±1 and 43.2±0.2 m²/g for Ce and Pr) and x-ray line-broadening. Theunaggregated, single crystal nature of the particles was confirmed bytransmission electron microscopy. Dopant concentration corresponded to75 dopant ions per 20 nm particle in the case of Ce-doping and 800dopant ions per 40 nm particle for Pr. As-grown powders were excited atlow current and voltage with an electron beam to record optical emissionspectra, and coherent back-scattering experiments at several wavelengthswere performed to measure l*(λ), the mean distance over which fieldspropagate before becoming directionally-randomized. To do so, loosepowder samples were lightly pressed into a shallow, disk-shaped recessmachined into an oxygen-free copper holder and placed in an ultrahighvacuum chamber operated at a pressure of 7×10⁻¹⁰ Torr. Electrons withenergies in the 1-10 keV range were lightly focused to spot diameters inthe range φ=2-7 mm on the sample. Luminescence was analyzed through aMgF₂ optical port with a 1 meter Czerny-Turner spectrometer. At variouselectron beam currents and at room temperature, the spectra of Pr³⁺:β-Al₂O₃ in FIG. 1 shows that red, green and blue emission lines areevident, and that the cathodoluminescence is whitish-yellow to the nakedeye.

Example 4

Aluminum tri-sec-butoxide (Aldrich) was purified by distillation undervacuum. In a N₂ Dri-Box, the purified aluminum tri-sec-butoxide, 250 g(1.015 mol) was slowly added to a stirred mixture of 155 g (1.039 mol)triethanolamine and 250 ml isopropanol. Within 2 h, the triethanolaminedisplaces the sec-butanol to form an air and water stable alumatrane.The resultant alumatrane in sec-butanol and isopropanol was diluted withethanol to create a 2 wt % residual ceramic solution. To this solution0.217 g (0.0005 mol) of cerium nitrate hexahydrate and 0.219 g (0.0005mol) of neodymium nitrate hexahydrate was added. The doped solution wasconverted to an ultrafine oxide by flame spray pyrolysis.

Example 5

The alumatrane of Example 4 in sec-butanol and isopropanol was dilutedwith ethanol to create a 1 wt % residual ceramic solution. To thissolution 0.217 g (0.0005 mol) of cerium nitrate hexahydrate and 0.219 g(0.0005 mol) of neodymium nitrate hexahydrate was added. The dopedsolution was converted to an ultrafine oxide by flame spray pyrolysis.

Example 6

The alumatrane of Example 4 in sec-butanol and isopropanol was dilutedwith ethanol to create a 8 wt % residual ceramic solution. To thissolution 0.217 g (0.0005 mol) of cerium nitrate hexahydrate was added.The doped solution was converted to an ultrafine oxide by flame spraypyrolysis. The degree of doping of particles of a given size from thisexperiment was calculated as follows.

Degree of doping of a selected ceria doped δ-Al₂O₃

Density of Al₂O₃ 3.98 g/cc

Unit weight 101.96128 g/mol

Particle Number of dopant diameter Weight of particle Units per particleatoms per particle 5 2.60E-19 1538 2 10 2.08E-18 12304 12 12 3.60E-1821261 21 18 1.22E-17 71756 72 20 1.67E-17 98431 98 22 2.22E-17 131012131 26 3.66E-17 216253 216 27 4.10E-17 242178 242 28 4.57E-17 270095 27030 5.63E-17 332205 332 50 2.60E-16 1537988 1538 70 7.15E-16 4220239 4220100 2.08E-15 12303903 12304

Example 7

In a 2 L flask, 25.00 g of indium acetate and 1.87 g of dibutyl tindiacetate were dissolved in 1 L of ethanol to create a 2 wt. % residualceramic solution. The solution was converted to an ultrafine oxide byflame spray pyrolysis. The fine powder, failing to contain dopant atoms,is not expected to lase.

Example 8

In a 2 L flask, 25.00 g of indium acetate and 1.87 g of dibutyl tindiacetate and 0.023 g of Pr(NO₃)₃6H₂O were dissolved in 1 L of ethanolto create a 2 wt. % residual ceramic solution. The doped solution wasconverted to an ultrafine oxide by flame spray pyrolysis. The powderobtained, when appropriately stimulated, for example by light, electronbeam, electron current or other means, is expected to lase.

Example 9

In a 2 L flask 0.1904 mol (145.87 g) of yttrium nitrate hydrate, 0.0177mol(15.912 g) of ytterbium nitrate hydrate and 0.0133 mol (11.780 g) oferbium nitrate hydrate were dissolved in 1250 g of ethanol to make a 4wt. % ceramic solution. This solution was then converted to a nano oxideceramic of composition; (Y_(0.86)Yb_(0.06)Er_(0.08))₂O₃ by the FSPprocess. The material was primarily the cubic phase with somemonoclinic. Particle sizes were typically 20 to 100 nm ave. particlesize depending on solution concentration used.

Green Emitting Particles Target (mol %) Actual (mol %) Y 0.86 0.863 Yb0.08 0.796 Er 0.06 0.0570

For the powders produced in example 6 the following analyses were done.

Specific surface area analysis (SSA) was conducted at 77k using aMicromeritics ASAP 2000 sorption analyzer (Norcross, Ga.) with N2 as theadsorbate gas. Samples were degassed at 395° C. for 4 h or until the outgas rate was <5 μm Hg/min. SSAs were calculated using the BET multipointmethod with five data points with relative pressures between 0.001 and0.20. The analysis was done using the software package supplied with theinstrument. The particle size was calculated from the SSA based onspherical non-aggregated particles. The surface area was determined tobe 55.0 m₂/g which corresponds to an average particle diameter of about27.4 nm based on the following table equating SSA with average particlesize.

SSA Particle diameter (nm) 10 151 20 75 30 50 40 38 50 27.4 55 25 60 2570 22 80 19 90 17 100 15 110 14 120 13

XRD line broadening. The doped δ-Al₂O₃ powder samples were analyzed bypowder X-ray diffraction using a Rigaku Rotating Anode Goniometer(Rigakeu Denki Co., Ltd., Tokyo, Japan). The powder was packed into anamorphous quartz specimen holder and placed into the goniometer. Scanswere measured from 10° to 60° 2θ at a scan rate of 2° 2θ/min using 0.05°2θ incrments and CuKα radiation operating at 40 kV and 100 mA. Peakpositions were compared with standard ICDD files to identify crystallinephases. For line broadening analysis, scans were measured from 33° to36° 2θ at a scan rate of 0.2° 2θ/min using 0.005° 2θ increments and CuKαradiation operating at 40 kV and 100 mA. The δ-Al₂O₃ peak centered at34.6 2θ was chosen for the calculations because it is fully resolvedfrom other peaks in the XRD pattern. This peak was compared to the26.54° 2θ peak of a single crystal quartz standard for Debye-Scherrercrystallite size calculations. An average particle size of 42.6 nm iscalculated-using the basic Debye-Scherrer equation. However when theanalysis was done using the software package supplied with theinstrument, which accounts for slit sizes, operating voltage and currentand uses a non-simplified form of the Debye-Scherrer equation, theaverage particle diameter is calculated to be 27.7 nm.

Assessment of Stimulated Emission

A wide variety of powders were prepared doped with various lanthanidemetals including: Er, Ce, Nd, Pr and Tu. As processed FSP n-δ-aluminadoped with 1000 ppm of Ce₃₊, φ=30, is illustrated in FIG. 7. Dopinglevels reached parts-per thousand to produce homogeneously dopedparticles despite the fact that the solubility of most lanthanide metalsin alumina is at the parts-per-thousand level. These particles areδ-alumina as determined by XRD powder patterns. These particles haveave. dia. of ≈30 nm as determined by BET analysis, Example 6. Thetypical particle sizes used for the lasing studies discussed below arein the 10-30 nm range as confirmed by both BET analyses and x-raydiffraction line broadening. The total number of dopant atoms rangedfrom about 50 to 500 per particle depending on the dopant, the matrixand the relative concentrations.

Electron Pumped Luminescence From Doped Oxide Nanopowders

Weak electronic excitation in doped oxide nano-particles readilygenerates stimulated emission at room temperature and coherentback-scattering measurements confirm that the particles are lossless onlength scales of both the scattering mean free path l and the transportmean free path l*. Even at the visible and ultraviolet wavelengths (λ)of the experiments, conditions adequate to produce electromagneticlocalization exist (l,l*<<,λ). Electron excitation can easily sustaincontinuous wave, infrared, blue light and ultraviolet laser action inthis regime.

The size distribution of the powders is broad, and accuratelylog-normal. For some of the experiments, Nd and Ce dopants wereincorporated into samples at various Nd/Ce ratios and concentrations ofroughly 240 dopant ions per particle of average diameter 27 nm. Theloose powders were lightly pressed into a shallow, disk-shaped recessmachined into an oxygen-free copper holder and placed in an ultrahighvacuum chamber operating at a pressure of 9×10⁻¹⁰ Torr. A steerable beamof electrons with energies in the 0.5-10 keV range was lightly focusedto a spot diameter between φ=2-7 mm on the sample, and luminescencethrough an optical port was analyzed with a 1 meter Czerny-Turnerspectrometer with a holographic grating blazed for 250 nm.

At 8 keV, a series of spectra was recorded versus electron beam currentat room temperature in Nd,Ce:Al₂O₃ nanoparticles (FIG. 1(b)). A broadbackground underlying all the rare earth spectra between 350-450 nmresembled the spectrum of the undoped host (not shown), suggesting thatit derived from intrinsic point defects in the alumina. At 1 μA (bottomtrace), the cathodo-luminescence revealed many sharp lines correspondingto the primary dopant Nd³⁺. A prominent line, at 25,000 cm⁻¹ (400 nm),was assigned to the ²F_(5/2)-⁴F_(7/2) transition of Nd³⁺. As theexcitation level was increased above 20 mA, three emission linesoriginating from the ²F_(5/2) state (at 25,000, 32,500 and 34,500 cm⁻¹)were quenched. Simultaneously, a fourth transition (²F_(5/2)-⁴F_(3/2))at 27,000 cm⁻¹ originating from the same (²F_(5/2)) upper state grewrapidly in intensity. In similar fashion, a spectral line at 23,000 cm⁻¹corresponding to the ²P_(1/2)-⁴F_(9/2), transition was quenched at 20μA, whereupon a second feature (²P_(1/2)-⁴I_(11/2) transition at 21,300cm⁻¹) originating from the same (²P_(1/2)) state was observed to growrapidly with increasing current.

The dramatic spectral redistribution of Nd³⁺ emission evidenced in FIG.1(b), as excitation current was increased, is consistent with the onsetof optical gain. As the result the possibility of achieving laser actionon dopant transitions in the samples is apparent. This, together withthe apparent absence of intense Ce³⁺ emission in the initial sample,prompted examination of powders with predominantly Ce³⁺ doping.

Spectra recorded from samples with heightened Cerium doping showedemission primarily from Ce³⁺, as indicated in FIG. 1(c). In this case, abroad feature centered at 28,000 cm⁻¹ dominated the spectrum. A smallshoulder was observed ˜2.500 cm⁻¹ higher in energy, consistent with theCe³⁺ (4f) ground state splitting, which for fluoride crystals isapproximately 2,100 cm⁻¹. D. J. Ehrlich, P. F. Moulton, and R. M.Osgood, Jr., “Ultraviolet solid-state Ce:YLF laser at 325 nm,” Opt.Lett. 4, 184-186 (1979). In many materials, the short wavelength(5d-²F_(5/2)) component of the 5d-4f inter-configurational transition ofCe³⁺ is higher intensity, than its long wavelength companion5d-²F_(7/2). D. J. Ehrlich, P. F. Moulton, and R. M. Osgood, Jr.,“Ultraviolet solid-state Ce:YLF laser at 325 nm,” Opt. Lett. 4, 184-186(1979); S. Mroczkowski and P. Doran, “Preparation of Rb₂NaYF₆:Ce³⁺ andCs₂NaYF₆:Ce³⁺—Prospects for tunable lasers of blue-green wavelength,” J.Less-Common Metals 110, 258-265 (185); A. A. Kaminskii, S. A. Kochubei,K. N. Naumochkin, E. V. Pestryakov, V. I. Trunov, and T. V. Uvarova,“Amplification of ultraviolet radiation due to the 5d-4finterconfigurational transition of the Ce³⁺ ion in BaY₂F₈,” Sov. J. Qu.Elect. 19, 340-342 (1989); J. F. Pinto, G. H. Rosenblatt, L. Esterowitz,V. Castillo and G. J. Quarles, “Tunable solid-state laser action inCe³⁺:LiSrAlF₆,” Electr. Lett. 30, 240-241 (1994); C. D. Marshall, J. A.Speth, S. A. Payne, W. F. Krupke, G. J. Quarles, V. Castillo and B. H.T. Chai, “Ultraviolet laser emission properties of Ce³⁺-doped LiSrAlF₆and LiCaAlF₆,” J.O.S.A. B11, 2054-2065 (1994); N. Sarukura, M. A.Dubinskii, Z. Liu, V. V. Semashko, A. K. Naumov, S. L. Korableva, R. Y.Abdulsabirov, K. Edamatsu, Y. Suzuki, T. Itoh, and Y. Segawa,“Ce³⁺-activated fluoride crystals as prospective active media for widelytunable ultraviolet ultrafast lasers with direct 10-ns pumping,”I.E.E.E. J. Select. Topics in Qu. El. 1, 792 (1995); B. Huttl, U.Troppenz, K. O. Veithaus, C. R. Ronda, and R. H. Mauch, “Luminescenceproperties of SrS: Ce³⁺,” J. Appl. Phys. 78, 7282 (1995). The relativelyweak shoulder at short wavelengths in FIG. 1(c) may indicate substantialresonant re-absorption at this wavelength by 4f gorund state ions orpoint defects.

The FIG. 1(c) spectra exhibit striking evidence of stimulated emissionfrom Ce³⁺. When the peak (or integrated) intensity of the, Ceultraviolet emission is plotted vs electron current, an abrupt change inslope is observed at room temperature (FIG. 2(c)). This is accompaniedby a 30%, narrowing of the emission spectrum. The sigmoidal linewidthvariation in the inset of FIG. 2(c) demonstrates this spectralnarrowing, which occurs at the same current level as the change inslope. These observations of stimulated emission with a threshold andspectral narrowing furnished strong evidence of continuous-waveultraviolet laser action on the 361 nm inter-configurational 5d-4ftransition of Ce³⁺.

Next, the scattering conditions in the alumina powder samples wereevaluated in coherent back-scattering experiments, using a simplescanning-detector apparatus. A stepper-motor-controlled translator wasused to scan a flat plate which pivoted about the sample mount. In thisway, a photomultiplier and filter assembly mounted on the plate werescanned angularly about the exact back-scattering direction. Probe lightat a wavelength of 488 nm, chosen to overlap one of the transitionsexhibiting gain, was selected from the output of an Ar ion laser using asmall monochromator, and directed after collimation to the samplesurface using a miniature turning mirror. By choosing the detectoraperture and translator step-size to correspond to the solid angle ofscattered light subtended by the turning mirror, only a few angularpositions near exact back-scattering were eclipsed by the mirror. Theresults are shown in FIG. 3. For Ce³⁺, a filtered ultraviolet Ar⁺ laserwith output at 363.8 nm was used. For Pr doped samples, or He—Ne laserat 632.8 nm was employed.

The data clearly show a broad back-scattering cone, as expected forsmall particles with a short transport mean free path l*. The mereobservation of the coherent scattering contribution near θ=0 indicatesthat the medium is lossless on the length scale of l*. E. Akkermans, P.E. Wolf, R. Maynard, and G. Maret, “Theoretical study of the coherentbackscattering of light by disordered media,” J. Phys. (Fr) 49, 77-98(1988). This is significant in itself, in that net optical gain wouldnot be favored in the presence of significant absorption. Additionallyhowever, it is possible to determine l* directly from analysis of theback-scattering cone at small angles. Accounting for internalreflection, we obtained a value of l*=144±31 nm was obtained from aleast squares fit to the data of FIG. 3(c), using the expression for<I(0)I(q)>/<I>² from J. X. Zhu, D. J. Pine, and D. A. Weitz, “Internalreflection of diffusive light in random media”, Phys. Rev. A44,3948-3959 (1991). This confirmed that our experiments were performed inan extreme scattering regime, where l*<<λ over the entire range ofwavelengths investigated.

Returning to the Nd³⁺ system, the dramatic spectral redistributionevidenced in FIG. 1(b), as excitation current was increased, isconsistent with the onset of stimulated emission on two emissionfeatures of Nd³⁺ above 20 μA. The initially dominant group of²F5/2-⁴F_(7/2) lines of Nd³⁺ at 25,000 cm⁻¹ as well as the²F_(5/2)-⁴I_(15/2) and ²F_(5/2)-⁴I_(13/2) lines, quenched suddenly asthe current was raised above this value, whereas the intense²F_(5/2)-⁴F_(3/2) feature originating from the same initial state grewrapidly at this point. This can only happen through the development ofoptical gain. Stimulated emission readily accounts for the quenching ofseveral radiative decay channels from the same initial state by the onetransition which experiences population inversion first. The same typeof spectral redistribution was observed for lines originating from the²P_(1/2) state, which has a large energy gap to lower states, tending topromote inversion. In this case, the intensity of the ²P_(1/2)-⁴I_(11/2)transition suddenly grew when the ²P_(1/2)-⁴I_(9/2) transition wasquenched, again above a current of 20 μA.

In FIG. 4, the peak intensity of emission at 469 nm is plotted versuselectron current for a more complete data set than depicted in FIG.1(b). An abrupt change in slope occurs at 10 μA, indicating the presenceof a threshold in the stimulated emission at this wavelength. Asindicated by the data in FIG. 2(d), very similar behavior occurs at 870nm. Because the spectral features are composite lines, consisting ofmany multiplet-multiplet transitions, no direct measurements of gainnarrowing on single lines were possible to supplement the course-grainintensity redistribution observations in the spectra of FIG. 1(b). Thedependence of emission intensity in the broad feature at 370 nm wasmonotonic and showed no indication of a threshold.

In summary, stimulated emission and laser action readily occur in oxidenanocrystals doped with rare earth ions when appropriate electronexcitation is provided and absorptive losses are negligible. Thesematerials are capable of continuous-wave ultraviolet laser action, bluelight laser action and infrared laser action in the strong scatteringregime on the 5d-4f inter-configurational transition of Ce³⁺ at roomtemperature and inter-configurational transitions of Nd³⁺ as describedabove. Under some conditions (by changing matrix, doping ion or byaddition of combinations of ions or by combinations of ions andmatrices) to produce other single wavelength and multiple wavelengthstimulated emission.

Coherent back-scattering results at 488 nm confirms that lightpropagation is lossless and the transport mean free path was very muchless than a wavelength. On this basis, doped dielectric nanophosphorsoffer potential for an entire family of novel, bright sources emittinglight uniformly over wide fields of view at ultraviolet, visible andinfrared wavelengths. Some important possible applications includenanolithography, sensors for atmospheric pollutants and toxic gases,sources for airborne and space communication devices, and novelmaterials for displays. By the term “thermodynamic concentration” ismeant the thermodynamically stable concentration limit.

What is claimed is:
 1. A process for the preparation of optically activemixed metal oxide powder having a mean particle size of less than 500nm, said process comprising pyrolyzing a mixture of at least one matrixmetal oxide precursor compound and at least one transition metal or rareearth metal dopant compound at a temperature in the range of 500° C. to2500° C. in the vapor phase and condensing to form a powder of mixedmetal oxides which exhibits stimulated emission of radiation, pulsedlaser action, or continuous laser action.
 2. The process of claim 1,wherein the particles of the mixed metal oxide powder have a meanparticle size less than 150 nm.
 3. The process of claim 1 wherein thematrix metal oxide comprises one or more oxides selected from the groupconsisting aluminum oxide, yttrium oxide, silicon oxide, zinc oxide, tinoxide, indium oxide, alkali metal silicates, alkaline earth metalsilicates, boron oxide, titanium dioxide, ytterbium oxide, and leadoxide.
 4. The process of claim 1 wherein the matrix metal oxide furthercomprises at least two oxides of main group metals.
 5. The process ofclaim 1 further comprising supplying the dopant metal compound in aconcentration greater than the thermodynamic solubility of the dopantmetal in the powder, and condensing to particles of powder having adopant metal concentration greater than the thermodynamic solubility inthe powder particles.
 6. The process of claim 1 wherein said step ofpyrolyzing comprises flame spray pyrolyzing a metal oxide precursordissolved in a flammable organic solvent.
 7. A powder produced by theprocess of claim 1 which exhibits stimulated emission of radiation inthe strong scattering regime.
 8. A solid state laser device comprisingat least one optically active mixed metal oxide powder having a meanparticle size less than 500 nm, said mixed metal oxide powderscomprising a matrix metal oxide, and a dopant metal oxide selected fromtransition metal oxides and rare earth metal oxides, optical feedback tosustain laser action provided by strong localization of light in thestrong scattering regime.
 9. The laser device of claim 8 wherein atleast one said matrix metal oxide is selected from the group consistingof aluminum oxide, yttrium oxide, silicon oxide, zinc oxide, tin oxide,indium oxide, alkali metal silicates, alkaline earth metal silicates,boron oxide, titanium dioxide, ytterbium oxide, and lead oxide.
 10. Alaser device of claim 8 in the form of a thin film.
 11. A laser deviceof any of claim 8 wherein the emitting material is not quantum confined.12. A laser device of claim 8 which has no optical cavity.
 13. A laserdevice of claim 8 wherein the light emitted is of a different wavelengththan the stimulating light.
 14. A laser device of claim 8 wherein thestimulating light is ultraviolet light, infrared light, or visiblelight, and the emitted light is white light.
 15. A laser device of claim8 wherein the stimulating light is infrared light, and the emitted lightis visible light or ultraviolet light.
 16. A laser device of claim 8where stimulation of the optically active mixed metal oxide powder is byan electron beam.
 17. A laser device of claim 8 wherein said devicecomprises at least two different optically active mixed metal oxidepowders which generate light of different wavelengths.
 18. A laserdevice of claim 8 wherein said matrix oxide is one or more of aluminumoxide, silicon oxide, zinc oxide, tin oxide, indium oxide, boron oxide,lead oxide, or titanium oxide, or is an alkali metal silicate oralkaline earth metal silicate.
 19. A laser device of claim 8 whereinsaid mean particle size is less than 150 nm.
 20. A laser device of claim8 wherein the mean particle size of the mixed metal oxide particles andtheir assembly is such that the mean scattering length and transportmean free path are less than the wavelength of light emitted.
 21. Alaser device of claim 8 wherein the light output of said device isincoherent.